Properties, Specific Resistance, and Hardness of Steel. 349 



of t was made. Before each observation, the water in the 

 receivers was well stirred. 



Galvanoscope. — The galvanoscope used was a very delicate 

 instrument of Sauerwald, provided with mirror and astatic 

 needle. The deflection of the latter was read off by mirror 

 and scale. As the telescope of this instrument stood side by 

 side with the telescope of the thermometers, both readings 

 could be conveniently made by the same observer. 



V. Determination of the Specific Resistance. 



For determining the specific resistance of steel rods, use 

 was made of a Wheatstone-Kirchhoff s bridge. An appro- 

 priate mercury commutator allowed the observer to inter- 

 change the unknown resistances without altering the value 

 of those belonging to the bridge proper. Thermoelectric dis- 

 turbances were avoided as far as possible by closing the cur- 

 rent (one Smee with large resistance) only for very short 

 intervals of time. Finally they were eliminated completely 

 by replacing the hydroelectromotive force by a Weber's 

 magneto-inductor *. The resistances of all rods were deter- 

 mined in terms of an arbitrary standard h (0*0312 Siemens 

 unit at 0°), chosen to correspond in magnitude with the un- 

 known resistances. The galvanometer used was the one already 

 mentioned above. 



As the resistances to be measured were all very small 

 (O'l to O'Ol Siemens unit), great care had to be taken to 

 exclude all disturbing resistances arising from insufficient 

 contact. Soldering could not be resorted to, as it was believed 

 that the ends would thereby have been annealed . The method 

 adopted was as follows : — The ends of the rods having been 

 well cleansed, were covered to about 1 centim. with a thin 

 adhesive film of galvanically deposited copper, which was 

 thereupon amalgamated (easily accomplished by plunging the 

 freshly covered part in mercury). The rod thus prepared was 

 then fixed, together with a glass rod in two corks, in a manner 

 similar to that employed in the case of the thermoelement, 

 and the whole, except the amalgamated ends, covered with a 

 thick coat of varnish. Suitable wooden cups provided with 

 horizontal and vertical apertures completed the connexion of 



* Pogg. Ann. cxlii. p. 418, 1871. The use of the magneto-inductor in 

 connexion with the bridge was suggested by Kohlrausch. This physicist 

 also showed that this method is applicable even when the resistances to 

 be determined are in the form of coils, the extra currents produced being 

 calculable. I found the application of great convenience, inasmuch as the 

 observer always has the needle of a delicate galvanometer completely 

 under his control. 



